Environmental and thermal barrier coating for ceramic components

ABSTRACT

A component comprises a silicon-based substrate; and a protective coating for the substrate. The protective coating includes tantalum oxide (Ta 2 O 5 ) and an additive for suppressing transformation from beta Ta 2 O 5  to alpha Ta 2 O 5 .

CROSS REFERENCE TO RELATED APPLICATONS

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 60/278,102, filed Mar. 23, 2001.

BACKGROUND

[0002] The present invention relates to protective coatings for ceramic materials.

[0003] Advanced turbomachines use silicon (Si)-based ceramics such as silicon nitride, silicon carbide, and their composites for hot-section components. Due to the high temperature capability of Si-based ceramics, those ceramic turbomachines operate at higher temperatures with minimum cooling and higher engine performance. However, at operating temperatures above 1200° C., the Si-based ceramics can be adversely affected by oxidation and water vapor present in the flow stream. Such a hostile engine environment results in rapid recession of Si-based ceramics parts.

[0004] U.S. Pat. No. 6,159,553 discloses the use of tantalum oxide (Ta₂O₅) as coating material on silicon nitride parts. A tantalum oxide coating of 2 to 500 microns in thickness can effectively protect the surface of silicon nitride parts from oxidation and reacting with water vapor at high temperatures. However, there are some limitations on pure tantalum oxide coating on Si-based parts:

[0005] 1. Ta₂O₅ undergoes a phase transformation from low temperature phase (beta-phase) to high temperature phase (alpha-phase) at about 1350° C., which may cause cracking in the coating due to the volume change occurred during the phase transformation.

[0006] 2. Ta₂O₅ is susceptible to grain growth at temperatures above 1200° C. Pronounced grain growth results in large grain microstructure, which reduces the mechanical strength of the coating and induces high local residual stresses in the coating, and causes the coating to spall off.

[0007] 3. Ta₂O₅ has a coefficient of thermal expansion (CTE) about 3×10⁻⁶° C.⁻¹, whereas silicon nitride has a CTE in the range of 3.-4×10⁻⁶° C.⁻¹ and silicon carbide (SiC) has a CTE in the range of 4-5×10⁻⁶° C.⁻¹ Since there is about 10 to 30% CTE mismatch between Ta₂O₅ and silicon nitride, and an even higher CTE mismatch between Ta₂O₅ and silicon carbide, residual stresses will develop in the Ta₂O₅ coating on Si-based ceramics. The residual stresses can limit the service life of the coating.

[0008] 4. A pure Ta₂O₅ coating has low fracture toughness, which may adversely affect the mechanical integrity and the lifetime of the coating during service due to foreign object impact and particulate erosion events.

[0009] With those limitations, a Ta₂O₅ coating on Si-based ceramics would not provide adequate protection for turbine engine applications in which the maximum temperature goes above 1350° C., thousands of thermal cycles occur, and greater than five thousand (5000) hour coating lifetime is required. It would be highly desirable to significantly improve the Ta₂O₅ coating to meet those stringent demands for advanced ceramic turbine engine applications.

SUMMARY

[0010] According to one aspect of the present invention, a component comprises a silicon-based substrate; and a protective coating for the substrate. The protective coating includes tantalum oxide (Ta₂O₅) and an additive for suppressing transformation from beta Ta₂O₅ to alpha Ta₂O₅.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 illustrates a first method of improving the crystalline structure of a coating composition.

[0012]FIG. 2 illustrates a second method of improving the crystalline structure of a coating composition.

[0013]FIG. 3 is a schematic view of component formed in accordance with the present invention.

DETAILED DESCRIPTION

[0014] The present invention relates to Ta₂O₅-based coatings which can effectively protect Si-based ceramic turbine engine parts even when repeatedly subjected to extreme temperatures during operation. The coating compositions have improved microstructural and high temperature stability as compared to pure Ta₂O₅ when utilized to protect Si-based ceramic parts in turbine engine applications.

[0015] Applicants have found that the grain growth rate of Ta₂O₅ is reduced by the addition of alumina at temperatures above 1400° C. It is believed that the Al₂O₃ solid solution in Ta₂O₅ changes the defect structure of Ta₂O₅ such that ionic diffusion rate is slowed and that the transformation of beta to alpha Ta₂O₅, which triggers exaggerated grain growth at temperatures greater than 1350° C., is suppressed by the presence of Al₂O₃.

[0016] Applicants have found that the sintering property of Ta₂O₅ is also improved with the addition of alumina. Pressed pellets containing between 1.0 to about 10 mol % of Al₂O₃ have shown noticeably higher density than pure Ta₂O₅ pellets sintered under the same condition. This improved sinterability is believed to be due to the reduction of Ta₂O₅ grain coarsening rate by the Al₂O₃ addition, and/or the enhancement of Ta ion lattice diffusion as the number of cation vacancies is increased by the diffusion kinetics due to the presence of Al ions.

[0017] The solid solubility of Al₂O₃ in Ta₂O₅ may be about 10 mol % at about 1500° C. Since alpha-Al₂O₃ has a CTE about 8×10⁻⁶° C.⁻¹, the CTE of a 10 mol % Al₂O₃-90 mol % Ta₂O₅ alloy would be about 3.5×10⁻⁶° C.⁻¹, which is 10% higher than the pure Ta₂O₅ and closer to the CTE of silicon nitride. When the amount of Al₂O₃ in Ta₂O₅ exceeds about 10 mol %, a second phase having the formula of AlTaO₄ forms that has a CTE about 5×10⁻⁶° C.⁻¹. As the alloy composition increases to 25 mol % Al₂O₃-75 mol % Ta₂O₅, the microstructure includes a mixture of Ta₂O₅-Al₂O₃ solid solution and the AlTaO₄ compound, and the CTE is about 4×10⁻⁶° C.⁻¹ which matches well with that of SiC. If the Al₂O₃ concentration exceeds 25 mol %, the CTE of the coating would become too high for application on Si₃N₄ substrate. For SiC and its composites having CTE in the range of 4-5×10⁻⁶° C.⁻¹, the Ta₂O₅ coating can incorporate up to 50 mol % Al₂O₃ so that the majority of the phase in the coating becomes AlTaO₄ and the CTE match very well with the substrate.

[0018] A variety of ceramic processing methods can be used to introduce and incorporate the additives into Ta₂O₅. As shown by the method 100 in FIG. 1, the process may start with a commercially available Ta₂O₅ powder (step 102), to which a desirable amount (about 1-50 mol. %) of additives are added (step 105). The additives or their precursors can be in the form of powders that require only simple (either dry or wet) mixing with the Ta₂O₅ powder (step 106). After mixing (and drying, if wet mixing in a liquid medium is performed) the mixture 120 is ready for coating operation 108. Optionally, the mixture is subject to calcination (step 110) in which the mixture is heat-treated to a temperature up to 1600° C., after which with or without milling or grinding (step 112), before the coating operation. Coating is then performed (step 108), as described below.

[0019] Referring to FIG. 2, an alternative method 113 of applying the additives includes starting with precursor compounds (either solids or liquids) containing the additive ions (step 114). The precursor compounds are dissolved in a solvent such as water or an alcohol 116 (step 116), mixed with Ta₂O₅ powder (step 118), (alternatively, the Ta₂O₅ powder can be dispersed in the solvent first, and added with the precursors), and then precipitated onto the surface of the Ta₂O₅ particles (step 120). After drying (step 122), calcination (step 122), and/or milling/grinding (step 124), the mixture is then ready for the coating operation 126.

[0020] The coating operation (step 108, 126) for applying the mixture created by either of the methods 100 or 113 may include plasma spray, sol gel, and chemical vapor deposition. Moreover, the coating can be formed by sintering pressed ingots or similar components at about 1350° C. for 1 to about 20 hours, and using Physical Vapor Deposition, (PVD) or Electron Beam Physical Vapor Deposition (EB-PVD) methods, the latter being well known in the field of thermal barrier coating on super alloy turbine engine parts. Both PVD and EB-PVD coatings have the benefit of forming a smooth surface, allowing strong bonding to the substrate, and uniform additive distribution.

[0021] The additive is not limited to an oxide of aluminum. The additive for the coating may include one or more of other oxides, compounds, or their precursors of Al, Hf. Si, Ln (rare earth including whole lanthanum series and Y) Mg, Mo, Ni, Nb, Sr, Ti, and/or Zr. Those additives may affect the CTE of tantalum oxide, mostly shifting it higher. An additive such as La₂O₃ can induce an acicular-grain-shaped second phase having the formula La₂Ta₁₂O₃₃, and thereby produce a duplex microstructure that results in a strong material with high toughness. Additional additives (e.g., nitrides, carbides, borides, silicides) can be introduced to further inhibit grain growth, modify CTE, and reinforce tantalum oxide. These additives result in lower grain growth, higher CTE and increased fracture toughness.

[0022]FIG. 3 shows a component 200 formed in accordance with the present invention. Component 200 includes a substrate 202 which may be formed of Si-based material such as a SiC—SiC composite material. A thermal protective coating layer 204 is applied to the outer surface of substrate 202 as described above.

[0023] The coating layer 204 may be formed of a mixture of Ta₂O₅ and one or more additives including Al₂O₃ or La₂O₃. The coating thickness can be accurately controlled by EB-PVD techniques and may vary between 0.5 to 10 mil.

[0024] During application, the additive(s) in the coating layer 204 may go into solid solution of form compounds with the tantalum oxide. Therefore, the amount of additive(s) in the finished component may be different than the amount of the starting material.

EXAMPLES Example 1

[0025] Compositions having 1, 10 and 25 mol % Al₂O₃ as the additive to Ta₂O₅ were prepared. In each batch, about 1 Kg of a commercial beta Ta₂O₅ powder was mixed with commercial Al₂O₃ powder in isopropanol in a milling jar for about 2 hours before drying. After drying was complete, the powder was sieved to classify the particle size to about 5 to 100 micron range in preparation for plasma spray coating. If the particle size was too fine, a calcining process was included to coarsen the particles. A coating of the above composition was then applied to coupons of silicon nitride and SiC—SiC composite substrates by an air-plasma spraying process. The silicon nitride coupons had an as-sintered surface on which the plasma coating was applied. Alternatively, a grit-blasted machine surface could have been utilized. The coupons were then degreased, and preheated to about 1000° C. by either a torch or furnace. The powder was then fed into a high velocity, high temperature plasma air flow. The ceramic powder became molten and subsequently was quenched and solidified onto the coupons. The coating thickness varied from about 2 to about 10 mil, or about 50 to about 250 microns. The coated samples were then subjected to cyclic furnace testing wherein each sample was held in the furnace at about 1315° C. for about 30 minutes, and then quickly removed from the furnace and quenched to about 200° C. in a stream of blowing air. The silicon nitride coupons coated with Al₂O₃ in the range of about 1 to 25 mol % survived about 100 hours and 200 cycles without spalling. X-ray diffraction showed the Ta₂O₅ remained in the beta phase.

Example 2

[0026] Compositions having 3, 4, 6 and 10 mol % La₂O₃ as the additive to Ta₂O₅ were prepared. In each batch, about 1 Kg of a commercial beta Ta₂O₅ powder was mixed with commercial La₂O₃ powder in isopropanol in a milling jar for about 2 hours before drying. After drying was completed, the powder was sieved to classify the particle size to about 5 to 100 micron range in preparation for a plasma spray coating. The composition was applied to coupons of silicon nitride and SiC—SiC composite substrate by air-plasma spray process. The silicon nitride coupons had as-sintered surfaces on which the plasma coating was applied. Alternatively, a grit-blasted machine surface could have been utilized. The coupons were then degreased, and preheated to about 1000° C. by either a torch or furnace. The powder was then fed into a high velocity, high temperature plasma air flow. The ceramic powder became molten and subsequently was quenched and solidified onto the coupons. The coating thickness varied from about 2 to about 10 mil, or about 50 to about 250 microns. The coated samples were then subjected to cyclic furnace testing wherein each sample was held in the furnace at 1315° C. for about 30 minutes, and then quickly removed from the furnace and quenched to about 200° C. by a stream of blowing air. The silicon nitride samples coated with La₂O₃ in the range of 3 to 6 mol % survived more than 1000 hours and 2000 cycles at 1315° C. The SiC—SiC samples having La₂O₃ in the range with 4, 6 and 10 mol % survived more than 2,000 hrs and 4,000 cycles. SEM examination showed needle-shaped La₂O₃—Ta₂O₅ precipitates on the coating surface. X-ray diffraction showed the existence of a second phase containing La, possibly the La₂Ta₁₂O₃₃ phase according to the phase diagram. These needle-shaped second phases distributed uniformly throughout the coating increased the fracture toughness and mechanical strength of the coating. They also increased the CTE of the coating such that the CTE mismatch between the coating and the substrate was significantly reduced, resulting in improved coating life performance as shown by repeated heating during the cyclic furnace testing.

Example 3

[0027] The SiC—SiC coupon coating was prepared with a 50-mol % Al₂O₃ addition in the same manner as Example 1 which survived the same cycle furnace testing for over 100 hours without spalling. After the testing, the coating has transformed to the AlTaO₄ phase with some residual Ta₂O₅. Silicon nitride coating coupons having coating compositions of 10 mol % Al₂O₃-90 mol % Ta₂O₅ survived 500 hours at 1315° C. and 1000 cycles without spalling. X-ray diffraction of the tested sample shows that the predominant phase in the coating remains the beta Ta₂O₅ with some AlTaO₄ phase.

Example 4

[0028] Two coating compositions, 1 mol % Al₂O₃-99 mol % Ta₂O₅ and 5 mol % Al₂O₃-95% Ta₂O₅, were heat-treated at 1450° C. for 2 hours. X-ray diffraction showed that the samples remained predominantly beta Ta₂O₅ after the heat treatment. In contrast, pure beta Ta₂O₅ completely transformed to alpha Ta₂O₅ after a heat treatment of 1 hour at 1450° C. Scanning electron microscope examination showed that the grain size for the 5 mol % Al₂O₃ coating composition fired at 1450° C. was significantly smaller than the pure Ta₂O₅ sample fired at the same temperature. The coating composition of 5 mol % Al₂O₃-95 mol % Ta₂O₅ was further heated at 1550° C. for 15 hours, and the Ta₂O₅ remained as beta phase after the heat treatment.

Example 5

[0029] Powders of two compositions, 7.5 mol % Al₂O₃-92.5 mol % Ta₂O₅ and 4 mol % La₂O₃-96 mol % Ta₂O₅, respectively, were pressed into cylindrically-shaped green parts and sintered at 1350° C. for 10 hours to form ingots for EB-PVD coating. Substrates of silicon nitride and SiC—SiC composites were loaded in a vacuum chamber and an electron beam was focused on an ingot of the material to be deposited. The electron bombardment resulted in high local heating on the coating material, which evaporated atomistically and condensed onto the part. Oxygen gas was bled into the system to compensate for the loss of oxygen from Ta₂O₅ during the evaporation. The coating was chemically bonded to the substrate. The substrate was preheated to 800-1200° C. to improve bonding with the deposited material. The coated silicon nitride and SiC—SiC parts having a 50 micron thick coating survived the above-described cyclic furnace testing at 1315° C. for over 500 hours and 1000 cycles.

[0030] Although the present invention has been described above with reference to specific embodiments, it is not to be so limited. Instead, the present invention is to be construed according to the following claims. 

We claim:
 1. A component comprising: a silicon-based substrate; and a protective coating for the substrate, the protective coating including tantalum oxide (Ta₂O₅) and an additive for suppressing transformation from beta Ta₂O₅ to alpha Ta₂O₅.
 2. The component according to claim 1, wherein coating includes a mixture of tantalum oxide (Ta₂O₅) and an additive chosen from the group consisting of Al, Hf, Si, Ln (rare earth including whole lanthanum series and yttrium) Mg, Mo, Ni, Nb, Sr, Ti, and Zr.
 3. The component according to claim 2, wherein the coating further includes an additive selected from the group consisting of nitrides, carbides, borides and suicides.
 4. The component according to claim 1, wherein the substrate is one of a silicon nitride substrate and a silicon carbide substrate.
 5. The component according to claim 1, wherein the additive includes aluminum oxide (Al₂O₃).
 6. The component according to claim 5, wherein the aluminum oxide is in the range of about 1-50 mol % during application of the coating.
 7. The component according to claim 5, wherein the aluminum oxide is based on starting material in the range of about 1-50 mol %.
 8. The component according to claim 1, wherein the additive includes La₂O₃.
 9. The component according to claim 8, wherein the La₂O₃ is in the range of about 1-10 mol % during application of the coating.
 10. The component according to claim 8, wherein the La₂O₃ is based on starting material in the range of about 1-10 mol %.
 11. A component, comprising: a substrate formed of silicon nitride or silicon carbide; and a thermal protective coating of crystalline composition on an outer surface of the substrate; and the thermal protective coating including a mixture of tantalum oxide (Ta₂O₅) and an additive of at least one of Al₂O₃ and La₂O₃.
 12. The component according to claim 11, wherein the Al₂O₃ is in the range of about 1-50 mol %.
 13. The component according to claim 11, wherein the La₂O₃ is in the range of about 1-10 mol %.
 14. The component according to claim 11, wherein a surface of the coating has needle-shaped La₂O₃—Ta₂O₅ precipitates.
 15. A method of protecting a silicon nitride (Si₃N₄) or silicon carbide (SiC) substrate against repeated thermal cycles at elevated temperatures, the method comprising: mixing an additive including an oxide, compound or its precursor chosen from the group consisting of Al, Hf. Si, Ln (rare earth including whole lanthanum series and yttrium) Mg, Mo, Ni, Nb, Sr, Ti, and Zr to a quantity of tantalum oxide (Ta₂O₅) powder; preheating the mixture; and applying the heated mixture to the substrate.
 16. The method according to claim 15, further comprising firing the substrate and applied mixture to form a solidified protective coating on the substrate having a thickness between 0.5 to 10 mil.
 17. The method according to claim 15, wherein aluminum oxide (Al₂O₃) in the range of about 1-50 mol % is mixed with the Ta₂O₅ powder.
 18. The method according to claim 15, wherein La₂O₃. in the range of about 1-10 mol % is mixed with the Ta₂O₅.
 19. The method according to claim 15, wherein the mixture is preheated to a temperature of about 1000° C. before applying the mixture to the substrate
 20. The method according to claim 15, further comprising heating the mixture to a temperature of about 1600° C. and then grinding the mixture before applying the mixture to the substrate. 